Low voltage non-linear electrical resistance elements and method of manufacture thereof



Oct. 12, 1965 F. JOHNSON 3,212,043

LOW VOLTAGE NON-LINEAR ELECTRICAL RESISTANCE ELEMENTS AND METHOD OFMANUFACTURE THEREOF Filed March 26, 1962 4 w 3; mun-man 5 fl ///l' r/INVEN TOR CR5 0 ERKZ K LAw RENCE UOHNSo V United States Patent 3,212,043LOW VOLTAGE NON-LINEAR ELECTRICAL RE.

SISTANCE ELEMENTS AND METHOD OF MAN- UFACTURE THEREOF Frederick LawrenceJohnson, Flixton, Manchester, England, assignor to Associated ElectricalIndustries Limited, London, England, a British company Filed Mar. 26,1962, Ser. No. 182,200 Claims priority, application Great Britain, Apr.11, 1961, 13,019/61 12 Claims. (Cl. 338-20) This invention relates tonon-linear electrical resistance elements, that is, resistance elementswhich have a nonlinear voltage-current characteristic.

Such resistance elements for use with voltages greater than, say, 25volts are kell known and usually consist of a quantity of discreteparticles of a resistance material such as silicon carbide theelectrical resistance of which varies with variation in applied voltage,in combination with a ceramic binder. One well known type is known underthe registered trade mark Metrosil and its electrical characteristic maybe expressed in the form:

I and V are the current through and voltage across the resistanceelement respectively, the index ,8 is characteristic of the materialfrom which the resistance element is made, and the constant K dependsupon the shape and size of the resistance element as well as on thematerial. In practice, the voltage at any current density isproportional to the thickness of the resistance element and the currentat any voltage gradient is proportional to the area of the resistanceelement. The most common form of this type of resistance element is adisc and the lower the voltage, the thinner the disc is required to be.

Resistance elements of this type are usually manufactured by pressing amixture of granules of silicon carbide or other such material and aceramic material into the form of a disc which is then sintered. Howeversuch resistance elements designed for low voltage applica tions arerather fragile. For example, for use at 25 volts the disc may be only0.02 thick. For very low voltage applications the disc will comprise asingle layer of a finite number of the granules and this precludes theuse of a ceramic binder on the grounds of extreme fragility.

Various solutions of this problem have been proposed which employbonding agents such as epoxy resins (which are electrical insulatingmaterials) in place of ceramic material. This has the disadvantage thatsome of the o granules may be encapsulated in the insulating material,thereby introducing an uncertainty into the manufacturing process andleading to uncontrolled variation in the electrical properties of theresistance element.

It is an object of the present invention to provide a nonlinearresistance element which is suitable for use with low voltages (e.g.voltages of the order of A to 5 volts) and which is robust and haselectrical properties that can be accurately controlled and reproducedduring manufacture.

According to the invention a non-linear resistance element ismanufactured by arranging between two electrically conductive surfaces arelatively hard single layer of granules of a resistance material theelectrical resistance of which varies with variation in applied voltage,applying pressure to cause at least one of said surfaces to undergoplastic flow suflicient to cause the granules to penetrate into it, andsecuring said surfaces against separation.

Also, according to the invention, there is provided such a non-linearresistance element in which a single layer ice of granules of aresistance material whose electrical resistance varies with variation inapplied voltage, is constrained between two electrically conductivesurfaces at least one of which has the ability to undergo plastic flowor application of pressure and has the granules penetrating into it.

In carrying out the invention the interstices between the granules maybe filled with other granules of smaller particle size to givemechanical stability to the resistance element.

In order that the invention may be more fully understood reference willnow be made to the accompanying drawing in which, on an enlarged scale:

FIGS. 13 are sectional views of respective exemplary constructions of anon-linear resistance element conforming to the invention.

Referring to FIG. 1, the resistance element there shown comprises afinite number of granules 1 of silicon carbide or other resistancematerial the electrical resistance of which varies with variation inapplied voltage, which are arranged as a single layer and have beenconstrained under pressure between two facing electrical conductingsurfaces presented by a pair of juxtaposed metal discs 2a, 2b. Each ofthese discs 2a, 2b is composed of soft conductive material such asaluminium or soft copper with the result that the applied pressure hascaused the granules 1 to penetrate into them. The edges of the lowerdisc 2b are bent over to enclose the granules 1 and to maintain themetal discs 2a, 2b in position; the top disc 2a and the rim of the lowerdisc 2b are electrically insulated from each other by an annularinsulating washer 4. Electrical contact with the resistance element thusformed can be made by conventional means such as spring contacts, tags,etc. (not shown).

To enhance the mechanical stability of the complete resistance element,the interstices between the granules 1 may, as shown, he filled withother granules 1, of smaller particle size, of either an electricallyinert material such as sand or molochite or possibly of a non-linearresistance material which may or may not be the same as that from whichthe principle granules 1 are constituted: in the latter case it iscontemplated that the smaller granules would affect the electricalproperties to only a small ex: tent, contributing mainly to mechanicalstability.

The resistance element shown in FIG. 2 also comprises a finite number ofgranules 1 of silicon carbide or other such resistance material whichare constrained as a single layer between two facing electricalconducting surfaces presented by a pair juxtaposed metal discs 2a, 211.However, in this construction the discs 2a, 2b are of rigid metaland'each has its conducting surface coated with a layer of soft solder 3into which the granules 1 have been pressed as a result of appliedpressure.

A detailed manufacturing procedure for the resistance element of FIG. 2is as follows:

One surface of each of two metal discs is coated with a layer of softsolder, the thickness of this layer being preferably less than half thesize of the granules which have been selected because of theirelectrical properties. To facilitate the attainment of a single layer ofgranules, one solder surface is smeared with a thin film of grease,preferably a silicone grease. The requisite number of granules aresprinkled on to this prepared surface and are temporarily retainedthereon by the grease film. The other solder surface is then placed incontact with these granules and the two discs pressed together, therebycausing the surface coatings of solder to undergo plastic flow at thepoints in contact with the granules so that the granules penetrate intoand are held by the solder. The depth of penetration can be controlledby either regulating the pressure applied to the discs or by restrictingthe thickness of the solder layer. For instance there may be employed athick solder layer with high pressure, a thick solder layer with lowpressure, or a thin solder layer with high pressure. The extent ofpenetration governs the area of contact between the granules and thesolder and serves as an additional control over the electricalproperties of the resistance element, i.e.-the greater the area ofcontact, the greater the current and heat capacity and the less the 13value at low currents. While maintaining the pressure between the twodiscs, the rim of one of them (which has a greater diameter than theother) is bent over to retain the two discs in position upon subsequentremoval of the pressure. As in the case of the resistance element ofFIG. 1, electrical insulation between the two discs is achieved by meansof an annular U- shaped washer of insulating material interposed betweentheir rims, while electrical contact with the resistance element can bemade by conventional means such as spring contacts, tags, etc.

In the resistance element of FIG. 3, foil or other deformable conductivematerial Sis used instead of solder as the, surface coatings: theresistance elements of FIGS. 2 and 3 and the method of making them arein all other respects the same. In both FIGS. 2 and 3 the intersticesbetween the granules 1 may be filled with other granules of smallparticle size (not shown) as described previously with reference to FIG.1.

.In each of the resistance elements described above, instead of bendingover the rim of one of the discs to ho d the. latter in position, nutand bolt, spring clips, or other clamping means may equally well beprovided for this purpose. Also, the discs of the resistance elementneed not necessarily be flat, being for instance of armate shapeinstead.

The term single layer of granules has been used throughout thespecification because the characteristics of a non-linear resistanceelement according'to the invention are determined by those granuleswhich contact both conducting surfaces. However, there may be somegranules which overlie others and the above term is therefore to beconstrued as including this possibility.

What I claim is:

1. In a method of manufacturing a non-linear resistance element, thesteps of providing for a single layer of discrete granules of aresistance material, the electrical resistance. of which varies with thevariation in applied voltage, arranging between two electricallyconductive surfaces, said layer of discrete granules so that saidelectricallyconductive surfaces engage oppositely facing surfaces of atleast a substantial number of said discrete granules in said layer,applying pressure to cause at least One of said electrically conductivesurfaces to undergo plastic flow sufiicient to cause the granules to beheld by penetration into said one of said electrically conductivesurfaces, and thereafter securing said surfaces against separation.

, 2. In a method of manufacturing a resistance element, the steps ofcoating one surface of each of two metal members with a conductivematerial susceptible to plastic flow under pressure, providing for alayer of discrete granules of a resistance material, the electricalresistance of which varies with variation in applied voltage, applyingsaid layer of discrete granules to the coated surface of one member, soplacing the coated surface of the other member in contact with thegranules that the coatings on said metal members engage oppositelyfacing surface portions of at least a substantiial number of saidgranules in said layer, pressing the two members together withsufficient force to cause the surface coatings to undergo plastic flowat the points of contact with the granules so that the granulespenetrate into and are held by the material of said coatings, andthereafter securing said members together in such manner that they aremutually insulated other than through the granules.

3. The method defined in claim 2 using soft solder for the coatings ofconductive material.

4. The method defined in claim 2 using metal foil for the coatings ofconductive material.

5. The method according to claim 1, including step of filling theinterstices between the granules with other granules of smaller particlesize.

6. The method defined in claim 5 wherein said granules of smallerparticle size are of electrically inert materlal.

7. A non-linear resistance element comprising a single layer of discretegranules of a resistance material whose electrical resistance varieswith variation in applied voltage said granules being constrainedbetween two electrically conductive surfaces at least one of which hasthe ability to undergo plastic flow on application of pressure and hasthe granules penetrating into it, said electrically conductive surfacesbeing so disposed as to engage oppositely facing surfaces of at least asubstantial number of :said granules in said layer.

8. The resistance element defined in claim 7 wherein the intersticesbetween the granules of said resistance material are filled with othergranules of smaller particle size to improve mechanical stability.

9. The resistance element defined in claim 8 wherein the granules ofsmaller particle size are of electrically inert material.

10. In a method of manufacturing a non-linear resistance elementparticularly suitable for use with relatively low voltages, the steps ofproviding for a single layer of discrete granules of a relatively hardresistance material whose electrical resistance varies with variationsin applied voltage, so confining said layer of said discrete granulesbetween two members having coextensive elec-. trically conductivesurfaces that said members contact oppositely facing surface portions ofat least a substantial number of said granules in said layers, exertingsuflicient pressure urging said members together to cause at least oneof said surfaces to undergo .plastic fiow sufiicient to enable saidgranules to penetrate into saidone of said surfaces to be held by one ofsaid members, and thereafter securing said members against separation.

11. A non-linear resistance element comprising two members havingcoextensive electrically conductive surfaces holding between them underpressure a single layer of discrete granules of an electrical resistancematerial whose electrical resistance varies with variation in appliedvoltage, said granules being harder than at least one of said surfacesand said pressure being suificient to cause penetration of said granulesinto said one surface, said granules in said layer being so disposedthat oppositely facing surfaces of a substantial number of said granulesin said layer engage said electrically conductive surfaces.

12. The resistance element defined in claim 11, wherein said granulesare silicon carbide.

References Cited by the Examiner UNITED STATES PATENTS 2/42 Grisdale.6/55 Heytow et al. a 338 -20

7. A NON-LINEAR RESISTANCE ELEMENT COMPRISING A SINGLE LAYER OF DISCRETEGRANULES OF A RESISTANCE MATERIAL WHOSE ELECTRICAL RESISTANCE VARIESWITH VARIATION IN APPLIED VOLTAGE SAID GRANULES BEING CONSTRAINEDBETWEEN TWO ELECTRICALLY CONDUCTIVE SURFACES AT LEAST ONE OF WHICH HASTHE ABILITY TO UNDERGO PLASTIC FLOW ON APPRISION OF PRESSURE